J . Org. Chem., Vol. 43, No. 14, 1978 2789
Structure and Reactivity of u,P-Unsaturated Ethers
Structure and Reactivity of a,&Unsaturated Ethers. 16. Electrophilic Addition of Benzenesulfenyl Chloride to a$-Unsaturated Ethers and Sulfides Kenzo Toyoshima, Tadashi Okuyama,* and Takayuki Fueno Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, J a p a n Received December 20,1977 The rates of addition of benzenesulfenyl chloride to a,p-unsaturated ethers and sulfides have been measured in CCl* a t 30 "C. Reactivities of alkyl vinyl ethers and sulfides increase with the electron donating ability of alkyl group, p * being -5.4 and -1.7, respectively. p-Alkyl and 0-methoxy substitutions enhance the reactivity of vinyl ether while tu-methyl substitution influences it little. Adducts are exclusively the Markownikoff type. With ethyl propenyl ethers, anti addition is slightly dominant over syn addition. It was concluded from these results that the rate-determining transition state resembles a symmetrically bridged sulfonium ion intermediate 23 and that an open carbonium ion mediates between the rate- and product-determining steps.
It is established that the addition of a sulfenyl halide to an olefin proceeds through an episulfonium ion intermediate to give stereospecifically an anti adduct.l One example of a nonstereospecific addition of an arenesulfenyl chloride to an olefin has recently been found with 1-(p-alkoxypheny1)propenes.* It was suggested that the rate-determining transition state closely resembles the bridged sulfonium ion even in the latter reaction. Enol ethers undergo electrophilic attack to give a carbonium ion intermediate stabilized by the direct conjugation with the alkoxy oxygen atom. This conjugative stabilization of an open carbonium ion might make the contribution of a bridged structure unnecessary in the transition state.3 In view of these considerations, we have investigated the addition of benzenesulfenyl chloride to various enol ethers as well as vinyl sulfides. Structures of the substrates studied here are shown in Tables 11-IV. Experimental Section Materials. Benzenesulfenyl chloride ( I ) was prepared from diphenyl disulfide and sulfuryl chloride according to the literature,* boiling at 70 "C (10 mm) [lit.449 "C (4 mm)]. Carbon tetrachloride and carbon disulfide were distilled from PzOj. 2,4-Dinitrophenylhydrazine of reagent grade (Wako) was used without further purification. Commercial ethyl ( 3 ) , n-hutyl(6),and isobutyl vinyl ethers (7) were distilled from LiAlH4. Methyl (2) and tert- butyl vinyl ethers (5) were prepared by the alcohol exchange of 3 and 6, respectively.s Isopropyl vinyl (4) and other alkenyl alkyl ethers (9-15) were obtained by the pyrolysis of an appropriate acetal as described previously.6 Preparations of phenyl vinyl ether (8): 1,2-dimethoxyethylene (16),*alkyl vinyl sulfides (17-20),9 and phenyl vinyl sulfide (21)'O were described before. Styrene (22) was distilled from tert-butylcatechol just before use. Geometric isomers were separated by the distillation through a spinning hand column, isomeric purity >96% by VPC. Kinetic Measurements. All the reactions were carried out at 30.0 f 0.1 "C in a CC14solution under pseudo-first-order conditions with a 50 times excess of olefin. Each stock solution of an olefin (-0.1 M) and 1 (-0.2 M) in CC14 was prepared by weighing. Concentration of 2 was determined spectrophotometrically after hydrolysis (A 285 nm, acetaldehyde). Three milli1:iters of an olefin stock solution was thermally equilibrated at 30 "C in a stoppered quartz cuvette inserted in a water-jacketted cell ho1de.r. Into the olefin solution was injected 30 pL of a stock solution of 1 wi;h use of a microsyringe. The reaction was monitored by the disappearance of 1,,A,( 390 nm) using a Shimadzu spectrophotometer 1JV-200. The fast reactions with 10-12 were followed with the use of a stopped flow spectrophotometer Union RA-1100. In this case, a stock hl in concentration. solution of 1 was 2 X Pseudo-first-ordei plots were linear over 80% reactions for all the runs studied. Rate constants are given as averages of a t least three measurements. Reaction of 1 with 3. A solution of 0.1 g of 3 in 0.3 mL of CS2 was placed in an NMR sample tube and cooled at -78 "C. To this solution
was added slowly 0.2 g of 1 in 0.2 mL of CS2. The temperature of the mixture was raised slowly under spinning on an NMR spectrophotometer JNM-4H-100. The spectra were recorded a t -60, -30, and 22 "C. The 'H NMR spectra showed a gradual formation of the single product with the disappearance of 3. The spectrum of the product was not changed after 1 week a t room temperature. The reaction at room temperature gave the same product spectrum (Table I). The reaction was also conducted in a greater scale (1,l g; 3,0.5 g; CC14,5 mL) and the reaction product was subjected to the acid-catalyzed hydrolysis. After 5 min of reaction a t room temperature, the reaction mixture was shaken with 5 mL of 1 N HC1 in 80% aqueous ethanol. Hydrolysis products were treated with 2,4-dinitrophenylhydrazine. There was obtained after recrystallization (95% ethanol) ca. 1 g (ca. 44% yield) of the hydrazone, melting at 94-96 "C. Anal. Calcd for C14H12N404S: C, 50.61; H, 3.61; N, 16.87. Found: C, 50.42; H, 3.56; N, 16.83. The results show that the hydrolysis product is phenylthioacetaldehyde. Reactions of 1 with 8,17, and 21. These reactions were carried out in an NMR sample tube a t room temperature. The IH NMR spectra of the reaction mixture showed the formation of the single adduct. The spectral data are given in Table I. The hydrolysis products of the adduct were identified as above. All the adducts gave only phenylthioacetaldehyde as an aldehydic product. Reactions of 1 with 9c a n d 9t. The reactions in an NMR sample tube were carried out in the same way as above. The reactions a t room temperature resulted in an essentially identical 'H NMR spectrum both from 9c and from 9t. The spectrum showed the formation of two isomeric adducts, the spectra (e) and (t) in Table I, in the ratio of about 4/6. The sulfenyl chloride 1 was added to the CS2 solution of 9 a t lower temperature. lH NMR spectra were recorded with raising temperatures. The reaction mixture obtained from 1 and 9t showed the 'H NMR spectra of changing ratio of the adducts (e) and (t); 7.5/2.5 a t -60 to -30 "C, 713 at 0 "C, and 4/6 a t 22 "C. On the other hand, the reaction between 1 and 9c gave the adducts (e) and (t) in the ratio of 317 at -60 to -30 "C which changed to 4/6 a t 22 "C. The mixture of the two adducts (of the ratio 4/6) was subjected t o the hydrolysis in the same way as above. The aldehydic product was isolated as 2,4-dinitrophenylhydrazonein ca. 60% yield, mp 10.5-106.5 " C (95% ethanol). Anal. Calcd for C15H14N404S: C, 52.03; H, 4.04; N, 16.18. Found: C, 51.94;H, 3.97; N, 16.20.The hydrolysis product must he tu-phenylthiopropionaldehyde.
Results Kinetics. The rates of addition of benzenesulfenyl chloride (1) t o various enol ethers as well as vinyl sulfides were mea-
sured in CC14 solution in the presence of a large excess of the latter substrate. Pseudo-first-order plots were linear over 80% reactions. The geometric isomer of alkenyl alkyl ethers, which may undergo possible isomerization, was carefully investigated in its kinetic behavior. Both cis and trans isomers showed excellent linearity in their first-order plots during about 4 halflives. This indicates that the geometrical isomerization does not take place during the reaction to affect the rate of addition. 1978 American Chemical Society
Toyoshima, Okuyama, and Fueno
2790 J . Org. Chem., Vol. 43, No. 14, 1978
Table I. NMR Spectra of the Adducts CeH5SCHzCHCl(XR) or CH&H(SC6H5)CHCl(XR)n
No. 3 8 17 21 9
Substrate Registry no.
6, ppm ( J ,Hz)
XR OCzH5 OC6H5 SCH3 SCeH5 OC2H5(e) (t)
66303-47-7 66303-48-8 66303-49-9 66303-50-2 66303-51-3 66303-52-4
CH3
CH2S or CHS
CHCl
1.40 d (7.0) 1.46 d (7.0)
3.33 d (4.5),3.36 d (8.3) 3.35 d (4.1),3.67d (7.4) 3.35 d (15.0) :3.33d (15) ,3.2-3.6m ,3.2-3.6m
R
5.48 dd (4.~58.3) 5.43 dd (4.1,7.4) 4.93 t (15.0) 5.08t (15) 5.42d (4.8) 5.62 d (2.1)
5.75q, 1.10t 2.10s 7.05-7.70m 1.09t, 3.6-4.0m 1.20t, 3.6-4.0m
Solvent: CC14. Table 11. Rate Constants for the Addition of l a to Vinyl Ethers, CHz=CHOR, at 30 "C in CCll No.
2 3 4 5 6 7 8 22
Registry no.
1O2k:2,
R
M-1
CH3 CZH5 i-C3H7 t -C4H9 n-C4Hg i-C4Hg C6H5 Styrene
107-25-5 109-92-2 18888-46-5 926-02-3 11 1-34-2 109-53-5 766-94-9 52601-97-5
s-l
3.39 10.6 33.6 143 9.80 8.21 0.294 0.778
The IH NMR spectra of the reaction mixture from czs- and trans-propenyl ethyl ethers, 9c and 9t, showed the formation of two isomers of the adduct. Both the isomers give a-phenylthiopropionaldehyde on hydrolysis. That is, the two isomeric adducts obtained must be stereochemical isomers (erythro and threo) of the Markownikoff-type adduct. CHJCH=CHOC2Hj 9c or 9t
+ C,HjSC1 1
-
C J S\ ,CH-CH CH
,OW C'1
eikthro and threo
Registry no. 931-59-9. CH,'
The pseudo-first-order rate constants k 1 were proportional to the olefin concentration ranging 0.04-0.15 M. That is, the reaction is second order in reactants. rate = k l [ l ] rate = h2[olefin][l]
(1)
The rates were measured in the presence of a small amount of a radical inhibitor, benzoquinone, but the rate contitants k z obtained were within experimental errors equal to those obtained in its absence. The addition must occur electrophilically but not through a free-radical mechanism. The second-order rate constants k2 are summarized in Tables 11-IV for vinyl ethers, substituted vinyl ether3, and vinyl sulfides, respectively. S t r u c t u r e of Adducts. The IH NMR spectra of the reaction mixtures with vinyl ethers, 3 and 8, and vinyl sulfides, 17 and 21, showed the formation of single product. Each of these reaction products was subjected to acid-catalyzed hydrolysis in 1 N HC1 aqueous ethanol. The hydrolysis products were treated with 2,4-dinitrophenylhydrazine.The hydrazone isolated was identified as that of phenylthioacetaldehyde in all the cases examined above. CH:=CHXR
X =OorS
+
HO
C,H,SCl
1
-
C,HiSCH2CH0
C,H,SCH,CH,
+
HC1
/ XR
+
The isomer of greater coupling constant of a CHCl signal (e) may be assigned to that of the erythro configuration and that of smaller coupling constant the threo configuration.11 Thus, the cis isomer 9c gave a slightly greater amount of the threo adduct (70%)at a lower temperature of -60 "C while the trans isomer 9t resulted in the preponderant formation of the erythro isomer (75%) a t lower temperature. That is, the anti addition is favored over the syn addition. At higher temperature, however, both 9c and 9t gave the same ratio of the erythro and the threo adducts of 416. The isomer ratio of the adducts obtained at lower temperature also changed to this ratio (4/6) on the rise of the temperature (to 22 "C), which indicates it to be a thermodynamic equilibrium ratio. Discussion The Effects of Alkoxy a n d Alkylthio Groups. The rate constants given in Tables I1 and IV show that alkyl vinyl ethers are 3-5 times more reactive than the corresponding sulfides. Such a reactivity difference found in the acid-cata, ~ Jrather ~ lyzed hydrolysis was as great as 102-104 t i m e ~ . ~The small difference observed for the present reaction must be due to a smaller contribution for the p r conjugation involving the 0 or S lone pair electrons in the transition state like 23. The carbonium ionlike transition state (24) in the hydrolysis owes
c1 RXH
Ph (2)
The product of the addition is no doubt exclusively the Markownikoff-type adduct. The 1H NMR spectra are consistent with this structure. In order to examine a possible rearrangement of the initially formed adducts during the addition, the reaction between 1 and 3 was followed carefully on an NMR spectrometer at lower temperatures of -60 "C to room temperature. No incipient signals other than those of the Markownikoff adduct were observed. Furthermore, the spectra were stable over 1 week a t room temperature.
(X
=0
23
or
S)
(X
=
0 or S) 24
its stability greatly to the p r conjugation. The p r conjugative stability is attained more effectively with the 2p orbitals of oxygen than with the 3p orbitals of the sulfur a t ~ m . Such ~J~ effects must be moderate in the transition state 23, in which a positive charge resides partly on the sulfenyl S atom.' Consequences of these factors are the greater reactivity of the
Structure and Reactivity of a,P-Unsaturated Ethers
J . Org. Chem., Vol. 43, No. 14, 1978 2791
Table 111. Rate Constants for the Addition of 1 to Ethers (at 30 "C in CC14)
R, R, Registry no.
No.
9c 9t 1OC
lot llc llt 12c 12t 13c 13t 14 15 16c 16t
Ri
0% k2,
Rz
M-1
4696-25-7 4696-26-8 4188-64-1 4188-65-2 10034-12-5 10034-13-6 4884-01 -9 1528-20-7 16969-28-1 16969-13-4 927-61-7 926-66-9 7062-96-6 7062-97-7
s-l
2.03 1.12 4.23 2.75 3.33 0.574 8.57 1.68 10.6 3.22 1.84 0.117 2.04 1.19
Table IV. Rate Constants for the Addition of 1 to Vinyl Sulfides, CH2=CHSR, at 30 "C in CC14 No. 17 18
19 20 21
Registry no.
R
1822-74-8 627-50-9 926-65-8 14094-13-4 1822-73-7
CH3 CZHS i -C3H7 t -C4H9 C6H5
102k2,
M-1
s-l
1.03 1.20 2.29 3.04 0.274
Table V. Relative Reactivities of Substituted Vinyl Ethyl Ethers in the Sulfenyl Chloride Addition and the AcidCatalyzed Hydrolysis No.
Substituent
Addition
HydrolysisU
3 H 1.0 1.0 15 CY-CHg 1.1 -103 9c CZS-P-CH~ 19 0.39 9t trans-P-CH? 11 0.12 14 $,P-(CH3)2 17 0.03 12c CLS-P-C~HS 81 0.35 12t trans-3-C ~ H s 16 0.09 16cb CCS-P-OCH3 60 2.0 x 10-1 16tb t ran5 - $-OCH 3 35 0.5 x 10-3 c a Reference 6. Relative to 2. Reference 8.
vinyl ethers in electrophilic reactions and the greater reactivity difference between the ether and sulfide in the hydrolysis. Phenyl vinyl ether (8) and sulfide (21) showed essentially the same reactivity. Similar results were found in the hydrolysis; 8 is only several times more reactive than 21.10 Involvement of the phenyl group in the conjugation may moderate the above stabilization effects of the 0 and S lone pairs. The rate constants of alkyl vinyl ethers and sulfides are plotted against Taft's ckvalues13 of alkyl groups in Figure 1. Both alkyl vinyl ethers and sulfides increase in their reactivities in the order CH3 < CzHs < i - C3H7 < t-C4Hg, p* being -5.4 and -1.7, respectively. The reactivity increases with the increasing electron release of the alkyl group. Similar trends are seen in the data of Table I11 with alkyl propenyl and butenyl ethers. These results are reasonably understood by the stability of a positively charged transition state 23, although the ground-state electronic structure deduced from the NMR spectral investigationsgJLL is not straightforward. The absolute magnitude of p is ;greater for the ethers than for the sulfides.
0 0.2 6" Figure 1. Correlations of the rate constants.he, with Taft's u* values: ( 0 )vinyl ethers; ( 0 )vinyl sulfides. -0.4
-0.2
Similar results were found in the hydrations of alkyl ethynyl etherslSand sulfides16 as well as in the hydrolysis of aryl vinyl ethers and sulfides.1° The difference in the efficiency of electronic transmission between the 0 and S atoms may be attributed to their lone pair electrons in 2p and 3p orbitals. Anomalies found in the hydrolysis of alkyl vinyl sulfides ( p * > 0)9 were not observed here in the sulfenyl chloride addition. The Effects of Vinyl Substitution on the Reactivity of Vinyl Ether. The effects of alkyl and alkoxy substitutions of the vinyl hydrogen are found in the data summarized in Table 111. The results are compared with those observed for the acid-catalyzed hydrolysis (Table V). A contrasting tendency between the two electrophilic reactions is apparent in Table V. Thus, the 0substituents enhance the reactivity in the sulfenyl chloride addition while they reduce the hydrolysis reactivity of vinyl ether.6 The a-methyl group has little effect on the former while it enhances greatly the latter.'; The reactivities in the hydrolysis were rationalized by the relative stabilities of an intermediate carbonium ion like 24.6 A @-methylsubstitution diminishes the hyperconjugative stabilization of 24, while an 0-methyl substitution enhances it.s A 0-methoxy group contributes greatly to the stabilization of the ground state but destabilizes the transition state by the inductive electron attraction.8 On the contrary, both the alkyl and alkoxy substitutions would increase the stability of the transition state like 23 of the sulfenyl chloride addition by the net electron donation to
2792 J . Org. Chem., Vol. 43, No. 14, 1978
Toyoshima, Okuyama, and Fueno
the double bond of vinyl ether. In the transition state, the charge transfer interaction between the double bond and the electrophilic sulfenyl sulfur atom must be important t o its stability.lS Small effects of a-methyl substitution may be due to the polarity of the double bond. The charge transfer interaction may be greater with the less polar electron-rich double bond of the p'-substituted vinyl ether. In the &3-disubstituted ether (14), a second methyl group seems to have little effect on the reactivity. This would be attributed to the concurrent inverse steric effect. Rate-enhancing effects of alkyl substitutions were previously noted with some alkene~.~~ With &monosubstituted vinyl ethers, cis isomers are 1.5-6 times more reactive than the corresponding trans isomers. The greater reactivity of cis isomer in electrophilic additions to olefins has been generally observed with a large variety of olefins and reactions12v20and is attributed to the favorable Coulombic interaction between the olefin and the electrclphile in the transition statc.12 The present results are new additions to the data of a previously observed trend. In conclusion, the reactivities of vinyl ethers and sulfides in the sulfenyl chloride addition can be rationalized hy assuming a symmetrically bridged transition state like 23.
=C. /H
+
PhSC1
Orientation a n d Stereochemistry of t h e Addition. The structure of adducts was determined by the hydrolysis experiments and by the lH NMR spectroscopy. All the adducts were exclusively of the Markownikoff type. Analysis of the adducts from a propenyl ether showed that the anti addition is favored at lower temperatures while the adducts rearrange to the thermodynamic mixture of the erythro and threo isomers a t room temperature. These results indicate that the rotation barrier of an intermediate is not high enough to attain the stereospecificity of addition. As the kinetic results suggested, the transition state of the rate-determining step resembles the bridged sulfonium ion 23. After this step, an open ion 24 is formed, and the product-determining transition state may be of unsymmetrically bridged structure. This results in the formation of regiospecific (Markownikoff) but nonstereospecific adducts. Whether trapping of the intermediate by chloride ion occurs with a bridged or open ion, 23 or 24, is not obvious at the present stage although the equilibrium concentration of 23 must be greater than 24. Similar observations have recently been made by Schmid The present and Nowlan2 with 1-(p-alkoxypheny1)propenes. results show more clearly the situation with olefins directly conjugated with an alkoxy group. Registry No.-Phenylthioacetaldehyde, 66303-55-7; p h e n y l t h ioacetaldehyde-DNP, 66303-53-5; n-phenylthiopropionaldehyde, 55064-96-5; a-phenylthiopropionaldehyde-DNP, 66303-54-6.
References a n d Notes
I
(1) For reviews, see (a) P. E . D.de la Mare and R. Bolton. "Electrophilic Additions to Unsaturated Systems", Elsevier, New York, N.Y., 1966, pp 166-173; (b) N. Kharasch. "Organic Sulfur Compounds", Vol. 1, Pergamon Press, New York, N.Y., 1971, pp 375-396; (c)R. C. Fahey, Top. Stereochem., 3, 63 (1968); (d) W. H. Mueller, Angew. Chem., lnt. Ed. Engl., 8, 482 (1969); (e) G. H. Schrnid and D. G Garratt in "The Chemistry of DoubleBonded Functional Groups", S.Patai, Ed., Wiley, New York, N.Y., 1977, OD 828-854. rr (2) (a) G. H. Schmid and V. J. Nowlan, J. Org. Chem.. 37,3086 (1972): (b) Can. J. Chem., 54, 695 (1976). (3) T. Okuyama, K. Ohashi. K. Izawa, and T. Fueno, J. Org. Chem., 39, 2255 11974). (4) W.H.'Mueller and P. E. Butler. J. Am. Chem. SOC.,90, 2075 (1968). (5) H. Yuki, K. Hatada, K. Nagata, and K. Kajiyama. Bull. Chem. SOC.Jpn., 42, 3546 (1969). (6) T. Okuyarna, T. Fueno, H. Nakatsuji, and J. Furukawa. J. Am. Chem. Soc., 89, 5826 (1967). (7) T. Fueno, I. Matsumura, T. Okuyama, and J. Furukawa, Bull. Chem. SOC. Jpn., 41, 818 (1968). (8) T . Okuyarna and T. Fueno, J. Polym. Sci., Part A- 1, 9, 629 (1971). (9) T. Okuyama, M. Nakada, and T. Fueno, Tetrahedron, 32, 2249 (1976). (10) T. Okuyama, M. Masago. M. Nakada, and T. Fueno, Tetrahedron, 33, 2379 (1977). (11) G. Dana, 0. Convert, and C. Perrin, J. Org. Chem., 40, 2133 (1975). (12) T. Okuyama and T. Fueno, J. Org. Chem., 39, 3156 (1974); part XV of this series. (13) R . W. Taft, Jr., in "Steric Effects in Organic Chemistry", M. S.Newman, Ed., Wiley, New York, N.Y., 1956, Chapter 13: J. Am. Chem. SOC.,79, 1045 ( 1957). (14) K. Hatada, K. Nagata, and H. Yuki, Bull. Chem. SOC.Jpn., 43,3195 (1970); B. A. Trofimov, G. A. Kalabin, V. M. Bzhesovsky, N. K. Gusarova, D. F. Kushnarev, and S.V. Amossova, Org. React. (USSR), 11, 367 (1974). (15) E. J. Stamhuis and W. Drenth, Red. Trav. Chim. Pays-Bas, 82, 394 (1963). (16) H. Hogeveen and W. Drenth, R e d Trav. Chim. Pays-Bas, 82, 375 (1963). (17) P. Salomaa, A. Kankaanpera, and M. Lajunen, Acta Chem. Scand., 20, 1790 (1966). (18) K. Izawa, T. Okuyarna, and T. Fueno. Bull. Chem. SOC. Jpn., 47, 1480 (1974). (19) Reference le, p 830. (20) R. Bolton in "Comprehensive Chemical Kinetics", Vol. 9. C. H. Bamford and C. F. H. Tipper, Ed., Elsevier, Amsterdam, 1973, Chapter 1. ~~~
*
,OR'
PhS,
li
23'
~~
